Deep structure of the northern Rio Grande rift beneath the San Luis basin (Colorado) from a seismic reflection survey: implications for rift evolution

A seismic reflection survey by Chevron across the San Luis basin (northern Rio Grande rift) and San Juan volcanic field of southern Colorado is reproc essed with extended correlation to search for basement structure. The trace of the main bounding fault of the basin, a high-angle normal fault against the Sangre de Cristo Range, can be correlated to a wide zone of dipping re flection fabric and soles out at lower crustal depths (26-28 km). The deepe r reflection fabric represent either broad extensional strain or pre-existi ng structure, such as a Laramide thrust system. The Sangre de Cristo boundi ng fault in San Luis basin does not sole out at mid-crustal depths but cont inues into the lower crust with a shallower dip. The basin architecture in the northern Rio Grande rift (San Luis basin) provides little if any eviden ce that the Sangre de Cristo bounding fault should flatten in a shallow lis tric fashion. This fault geometry is quite similar to the high-angle boundi ng fault in the Espanola basin but contrasts with less deeply-rooted faults in the Albuquerque basin in the central Rio Grande rift. Deeper soling out of the Sangre de Cristo bounding fault could be due to less extension in t he northern Rio Grande rift and/or greater strength of the Lithosphere comp ared to the central Rio Grande rift. Unequivocal Moho reflections beneath t he San Luis basin cannot be identified, probably due to limited signal pene tration or a gradational nature of the Moho. The majority of rift-related m ovement observed on the Sangre de Cristo bounding fault is post-Eocene. Eit her the western margin of the basin is marked by a tight monocline or a low -angle normal fault. (C) 1999 Elsevier Science B.V. All rights reserved.